1. Field of the Invention
The invention concerns an intervertebral prosthesis according to the precharacterizing portion of Patent Claim 1.
2. Description of the Related Art
Three-part intervertebral prosthesis are known, for example from EP 0 176 728 A1 or WO 02/08018 A1, comprised of two prosthetic end plates or cover plates, between which a prosthetic core is provided. The prosthetic end plates are comprised of a circular shaped or elliptical shaped disk, which exhibits a hemispherical recess on one side. The prosthetic core is in the shape of a circular disk, exhibiting on both sides respectively an axially disposed hemispherical segment and, running around the outer edge, an axially projecting ring segment. In the assembled condition of the intervertebral prosthesis the hemispherical segments of the prosthetic core lie in the hemispherical recesses of the two prosthetic end plates.
If the intervertebral prosthesis is introduced into the intervertebral space as replacement for a defective disk, which had been removed during an operation, then the two prosthetic end plates are respectively attached to respectively one vertebra and the prosthetic core is introduced between the prosthetic end plates. The cooperation of the hemispherical segments with the hemispherical shaped recesses makes possible a rotational movement of the two adjacent vertebrae.
Also known from, for example, WO 02/080818 A1 are three-part intervertebral prosthesis with a prosthetic core, which exhibits only one hemispherical segment and in place of the second hemispherical segment exhibits a flat segment. This flat segment is fixed into a correspondingly shaped recess in the one prosthetic end plate, since the movement of the one hemispherical segment in the hemispherical shaped recess is sufficient for a tilt and rotation movement of both prosthetic end plates relative to each other.
The described intervertebral prosthesis makes possible a rotation movement of the two vertebral bodies connected to each other via the prosthetic end plates about a point of rotation, which lies in the axis of the rotation-symmetric prosthetic core. During rotation and movement of an upper body with a healthy vertebral column however the rotation point does not, as a rule, lie in the center of the vertebral disk, but rather outside of the intervertebral space. The intervertebral prosthesis according to the state of the art thus exhibits a defect, of not optimally simulating the natural movement of the vertebral column.